Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers, and the power amplifier may often be the primary consumer of power in a personal audio device, and thus, may have the greatest effect on the battery life of the personal audio device. In devices having a linear power amplifier for the output stage, power is wasted during low signal level outputs, because the voltage drop across the active output transistor plus the output voltage will be equal to the constant power supply rail voltage. Therefore, amplifier topologies such as Class-G and Class-H are desirable for reducing the voltage drop across the output transistor(s) and thereby reducing the power wasted in dissipation by the output transistor(s).
In order to provide a changeable power supply voltage to such a power amplifier, a charge pump power supply may be used, such as that disclosed in U.S. patent application Ser. No. 11/610,496 (the “'496 Application”), in which an indication of the signal level at the output of the circuit is used to control the power supply voltage. The above-described topology may raise the efficiency of the audio amplifier, in general, as long as periods of low signal level are present in the audio source. Typically in such topologies, a plurality of thresholds define output signal level-dependent operating modes for the charge pump power supply, wherein a different supply voltage is generated by the charge pump power supply in each mode. In traditional approaches, the various thresholds are set for a worst-case scenario of the power amplifier (e.g., load impedance, process, temperature, etc.), such that in each mode, the power supply voltage is enough to provide a sufficient voltage headroom in order to prevent clipping of the output signal generated by the power amplifier. However, because a worst-case scenario is assumed in such approaches, when the worst-case scenario is not present (e.g., the load impedance differs from the worst-case load impedance), the power supply voltage provided by the charge pump power supply in some modes may be well in excess of that needed to provide sufficient voltage headroom, thus causing power inefficiency.
Therefore, it would be desirable to detect a value of a load impedance, so that the detected value may be used control a charge-pump power supply that supplies power to an audio power amplifier circuit for a consumer audio device, in which the efficiency of the audio output stage is improved.
Detecting the value of a load impedance may also provide other advantages in addition to control of a charge pump power supply.